1284 THE PIGMENT FACTOR CHAP. 32 



therefore sometimes called, rather awkwardly, the "fluorescence mechanism." We will 

 use the terms "fast" and "slow" migration, keeping in mind that the time yardstick by 

 reference to which the two cases are distinguished is the period of intramolecular vibra- 

 tion, about 10 ~" sec. 



Theoretical estimates indicate, and experimental results confirm, that 

 the exchange of excitation energy between resonating molecules can occur, 

 in a condensed system, not only in actual collisions or (to use a term more 

 appropriate for such systems) "encounters," but also while these mole- 

 cules are separated by a solvent layer of several molecular diameters. The 

 occurrence of such "remote" transfers was first derived from observations 

 of the "concentration depolarization" of fluorescence in dyestuff solutions. 

 When fluorescence of a dyestuff is excited by polarized hght, the fluorescent 

 light is found to be more or less strongly polarized. To explain this we 

 have to assume that excitation by polarized light occurs preferentially in 

 pigment molecules having a certain orientation, and that, in viscous media, 

 this orientation is not, or not completely, lost by thermal agitation in 

 the time between excitation and re-emission, thus resulting in an at least 

 partly polarized fluorescence. The degree of polarization of the fluores- 

 cent light proves to be a function of concentration, being highest in very 

 dilute solutions; this is the phenomenon of "concentration depolari- 

 zation." This depolarization occurs without change in the absorption or 

 fluorescence spectrum, the yield of fluorescence, or its life-time. It there- 

 fore cannot be attributed to dimerization (or generally, polymerization) of 

 the dyestuff molecules or ions. The alternative is then between attributing 

 depolarization to kinetic encounters, or to "remote" interactions between 

 dyestuff molecules. Probably, both phenomena occur. Perrin (1932), 

 Vavilov (1942-1950) and Forster (1946-1948, 1951) have been particularly 

 concerned with the remote interaction. The following observation speaks 

 in favor of this interaction as the main or only source of depolarization. The 

 concentration at which depolarization reaches 50% is (in the case of fluores- 

 cein solution in glycerol) of the order of 10 ~^ mole/Hter. Offhand, this 

 seems sufficiently high for kinetic encounters to produce the observed ef- 

 fect; however, if encounters were actually responsible, the concentration 

 of the dyestuff required for a certain degree of depolarization would in- 

 crease with increasing viscosity of the medium. No observations of such 

 a dependence have been made; furthermore, it was found that concentra- 

 tion quenching of the fluorescence of the same dye in sugar-glycerol solu- 

 tions (which occurs in a much higher concentration range, 10~^ to 10~^ 

 mole/1.) actually is independent of viscosity. The quenching thus appears 

 to be a function of the average mutual distance of the pigment molecules 

 (which is independent of viscosity) rather than of the frequency of their 

 encounters (which decreases with increasing viscosity). A fortiori, this 



